Bump pyyaml from 5.1 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1...5.4)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -17,7 +17,7 @@ numpy==1.16.3
 pbr==5.1.3
 protobuf==3.7.1
 pymagnitude==0.1.120
-PyYAML==5.1
+PyYAML==5.4
 scipy==1.2.1
 six==1.12.0
 tensorboard==1.13.1